Individual behavior weighs more heavily on the course of an epidemic than we often assume. According to new research from the Politecnico di Milano, even a relatively small share of people who disregard health guidelines can amplify transmission and push infection peaks forward in densely populated areas. The study, published in Proceedings of the Royal Society A, explored how non-compliance affects contagion patterns in cities.

Using real contact-network data and a mathematical model distinguishing between “compliant” and “non-compliant” citizens, researchers simulated the spread of an epidemic in Turin, Milan, and Palermo. The results show that infection peaks rise higher and arrive earlier when even a minor portion of the population ignores preventive measures, especially when the disease’s transmissibility is moderate. When these individuals are concentrated in specific neighborhoods, local hotspots develop, making containment efforts far more difficult.

The study argues for closer monitoring of behavioral patterns and for public-health strategies that adapt to the social geography of each city, improving the effectiveness of interventions during future outbreaks.